Flow over Ahmed body with Spoiler

Objective:

• Change in coefficient of drag and lift with change in spoiler angle.

Introduction:

• Fuel consumption and High speeds are two of the major concerns of automobile makers.
• Aerodynamics plays a major role in solving the above two problems.
• Force exerted by a flowing fluid on a stationary object can be divided into two components.
• Drag force: Component of force in the direction of motion.
• Lift force: Component of force perpendicular the direction of motion.

• Drag force is further governed by Friction and Pressure drag. For different types of bodies, one of the two dominates.
• Streamlined body: Body whose surface coincides with the streamlines, when the body is placed in a flow. Separation of flow takes place at its trailing edge.
• Bluff body: Body whose surface does not coincides with the streamlines, when the body is placed in a flow. Separation of flow takes place much ahead of its trailing edge.
• Drag force(Fd)= (Cd*(density)*(area)*(velocity^2))/2
• Drag coefficient(Cd) plays a major role here.
• Lift force is created due to negative pressure created on the upper part of the airfoil. Streamlined body.
• Lift force(Fl)= (Cl*(density)*(area)*(velocity^2))/2
• Lift coefficient(Cl) plays a major role here.

Spoiler:

• Spoiler shape is same as airfoil but the trailing edge is as shown.
• Spoiler is added to provide more down force. So it is opposite to lift force. This increases the stability of the car.
• In case of an airfoil, Velocity above the airfoil is more as compared to down, therefore pressure below the airfoil is more as compared to above, hence lift is observed.
• In case of a spoiler, Velocity above the spoiler is less as compared to down, therefore pressure above the airfoil is more as compared to below, hence negative lift or down force is observed.
• Hence with addition of spoiler, Lift coefficient decreases.

Geometry:

• Design a rear spoiler at different angles(-15 degree to 15 degrees).
• Prepare an outer enclosure for base geometry.
• Prepare an inner enclosure depending upon the wake zone size.
• Use split body to split the final geometry into half to refine mesh in student version.

Figure-1 Ahmed body with rear spoiler

Figure-2 Ahmed body with low angle rear spoiler

Figure-3 Ahmed body with high angle rear spoiler

Figure-4 Final geometry

Mesh:

• Use Hexa elements for outer enclosure for better accuracy.
• Inflation layer is introduced around Ahmed body to capture lift and drag coefficient accurately. Total thickness of 7.952 mm is given using Y+ value of 50.

Figure-5 Basic mesh

Figure-6 Inflation layer

Figure-7 Spacing between layers

Mesh Quality

Setup:

• Pressure based solver is used. Steady flow and absolute velocity is taken.
• K-epsilon, Standard wall functions model is taken.
• Air is taken as the working fluid.
• Inlet velocity of air is taken as 25m/s.
• Reference values: Area=0.055 m^2, Length=1.044, Velocity=25 m/s.
• Hybrid Initialization is done and solution is run for 350 iterations.
• Coefficient of drag and lift are calculated at car wall.

Case-1 Spoiler angle: 0 degrees

• Cd=0.467
• Cl=0.304

Figure-8 Residual

Figure-9 Cd plot

Figure-10 Cl plot

Figure-11 Y plus plot

Figure-12 Velocity vectors

Figure-13 Streamlines

Case-2 Spoiler angle: -15 degrees

• Cd=0.432
• Cl=0.357

Figure-14 Residual

Figure-15 Cd plot

Figure-16 Cl plot

Figure-17 Y plus

Figure-18 Velocity vectors

Figure-19 Streamlines

Case-3 Spoiler angle: 15 degrees

• Cd=0.519
• Cl=-0.136

Figure-20 Cd plot

Figure-21 Cl plot

Figure-22 Y plus

Figure-23 Velocity vectors

Figure-24 Streamlines

Figure-25 Cd and Cl vs Theta

Conclusion:

• Aerodynamic lift (Cl) decreases nearly linearly with the inclination angle of the spoiler.
• Aerodynamic drag (Cd) increases with the inclination angle of the spoiler.
• To avoid the formation of vortices Louvers are added with plate end cuts. These create vortices in opposite direction and hence reduces drag.